1. Field of the Invention
The present invention relates to a microcomputer and a non-contact IC card using the same, and, more particularly, to a technology capable of reducing their power consumption.
2. Description of the Related Art
FIG. 4 is a schematic view which illustrates a conventional non-contact IC card. A ROM 2A, a RAM 2B, a transmitting circuit 3, and a receiving circuit 4 are respectively connected to a CPU 1 as a microcomputer 5. A data transmitting antenna 6 and a data receiving antenna 7 are respectively connected to the transmitting circuit 3 and the receiving circuit 4 of the microcomputer 5, while a battery 8 and an oscillator 9 are connected to the CPU 1. In order to improve environmental resistance, the IC card is fully sealed with resin or the like.
Power supply voltage is supplied to the CPU 1 from the battery 8 and clock signals are supplied from the oscillator 9 so that the CPU 1 acts in accordance with a program previously stored in the ROM 2A. The IC card is capable of transmitting and receiving data from external devices by using electromagnetic waves. When data are received, external electromagnetic waves are received by the data receiving antenna 7 and demodulated by the receiving circuit 4 into data which are supplied to the CPU 1. The data thus supplied to the CPU 1 are processed and stored in the RAM 2B if necessary. On the other hand, when data are transmitted, data are output from the CPU 1 to the transmitting circuit 3 in which a carrier wave is modulated by the data. The modulated carrier wave is transmitted through the data transmitting antenna 6.
In general, the IC card is waiting for external electromagnetic waves, and the operations of the CPU 1, ROM 2A, RAM 2B, and the transmitting circuit 3 in the microcomputer 5 are stopped in order to prevent electric power consumption from the battery 8. However, since it is always necessary to always supervise the external electromagnetic waves, the operation of the receiving circuit 4 cannot be stopped. When electromagnetic waves are received by the data receiving antenna 7 in the above-described waiting state, the operation of the CPU 1 is restarted by the receiving circuit 4, and, in synchronization, the ROM 2A, the RAM 2B, and the transmitting circuit 3 are also operated. As a result, sequential processing such as data processing, data storage, and data transmissions take place, and then the IC card is again brought into the waiting state.
FIG. 5 is a schematic view which illustrates the receiving circuit 4. A capacitor 11 is connected in parallel with the data receiving antenna 7 as a resonant circuit 12 for receiving electromagnetic wave of a predetermined frequency. On the other hand, resistors 13 and 14 for determining a reference voltage are connected between a power source and ground, the resistors 13 and 14 being connected in series. A differential circuit includes P-channel transistors 15 and 16 to which P-channel transistors 17 and 18 are respectively connected, the P-channel transistors 17 and 18 being arranged to shift the level of the input voltage generated in the resonance circuit 12 and the level of the reference voltage generated by the resistors 13 and 14, respectively. A current mirror circuit including N-channel transistors 19 and 20 is connected to the differential circuit as a load. The P-channel transistors 21, 22, and 23 are provided for supplying bias currents to the transistors 17, 18 and the transistors 15 and 16 of the differential circuit.
The levels of the input voltage generated in the resonance circuit 12 in accordance with the received electromagnetic wave and the reference voltage generated by the resistors 13 and 14 are shifted in the transistors 17 and 18, respectively. The thus level-shifted voltages are supplied to the differential circuit to be compared with each other. As a result, an output signal corresponding to the received electromagnetic wave is output through a junction A of the transistors 15 and 19 to the CPU 1.
The operation of the receiving circuit 4 cannot be stopped even if the IC card is in a waiting state. Bias currents are always allowed to pass through the transistors 15 to 18 via the transistors 21 to 23 and a current is always allowed to pass through the resistors 13 and 14 in order to obtain the reference voltage. If the bias current to be supplied to the transistors 15 to 18 is reduced, the frequency characteristics of the receiving circuit 4 deteriorate. Therefore, it is necessary to pass a bias current of a certain high level through the transistors 15 to 18.
As a result, a problem concerning excessive power consumption of the IC card and another problem concerning short durability of the battery 8 arise. In particular, in the case where the battery cannot be replaced, the life of the IC card is shortened.